JEE Main · 2019 · 08 Apr 2019 · Shift 1 · d and f Block Elements

A lanthanoid ion is coloured only when a partly filled $4f$ shell allows f-f transitions. Here ${\mathrm{La}}^{3+}$ is $4f^0$, ${\mathrm{Gd}}^{3+}$ is the half-filled $4f^7$ and ${\mathrm{Lu}}^{3+}$ is the filled $4f^{14}$, all stable, transition-free and so colourless. Only ${\mathrm{Sm}}^{3+}$, with $4f^5$, has the partly filled shell needed for f-f transitions and is therefore coloured.